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Prototype safer AisBitset access wrapper #175

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Prototype safer AisBitset access wrapper #175

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d5d3129
Prototype safe wrapper for AisBitset accesses
jamtho Dec 9, 2017
9637dbc
Convert Ais20 and Ais26 ctrs to use the new safer AisBitset wrapper
jamtho Dec 9, 2017
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34 changes: 34 additions & 0 deletions src/libais/ais.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -234,7 +234,41 @@ const AisPoint AisBitset::ToAisPoint(const size_t start,
return AisPoint(lng_deg / divisor, lat_deg / divisor);
}

int AisBitset::GetValue(BitGetter getter) const {
assert(getter.target);
assert(getter.start + 1 <= GetNumBits());

if (getter.start + 1 > GetNumBits()) {
return -1;
} else {
*getter.target = operator[](getter.start);
return 0;
}
}
int AisBitset::GetValue(StringGetter getter) const {
assert(getter.target);
assert(getter.start + getter.len <= GetNumBits());

if (getter.start + getter.len > GetNumBits()) {
return -1;
} else {
*getter.target = ToString(getter.start, getter.len);
return 0;
}
}
int AisBitset::GetValue(UIntGetter getter) const {
assert(getter.target);
assert(getter.start + getter.len <= GetNumBits());

if (getter.start + getter.len > GetNumBits()) {
return -1;
} else {
*getter.target = ToUnsignedInt(getter.start, getter.len);
return 0;
}
}


// static private

void AisBitset::InitNmeaOrd() {
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57 changes: 57 additions & 0 deletions src/libais/ais.h
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Original file line number Diff line number Diff line change
Expand Up @@ -411,6 +411,50 @@ class AisBitset : protected bitset<MAX_BITS> {
// Visible for testing.
static bitset<6> Reverse(const bitset<6> &bits);

// Safe batch fetcher for multiple values stored within one AisBitset.
// Provide a list of 'getter' structs (defined below) to define the values to
// fetch and the addresses to write them to.
// These should be in start-position ascending order to preserve
// GetRemaining() etc.
// Note that GetValue() dies on an assert if an out of bounds request is made
// and they're compiled in.
template <class... Getters>
AIS_STATUS GetValues(Getters... getters) const {
// NB standard states order of evaluation in initlist is 0, 1, 2...
int rets[] = {GetValue(getters)...};
for (auto r : rets) {
if (r)
return AIS_ERR_BAD_BIT_COUNT;
}
return AIS_UNINITIALIZED;
}

// AisBitset sub-sequence 'Getter' objects for use in GetVal()/GetVals().
// The struct type states what type of value you want to interpret the
// sub-seq as, e.g. UIntGetter fetches unsigned integers (as ints).
// Their member variables define where the sub-seq is in the AisBitset, using
// the same numbering scheme as ToUnsignedInt() etc above.
// 'target' is an out parameter pointing where you want to write the value.
struct BitGetter {
bool *target = nullptr;
const int start = 0;
explicit BitGetter (bool *t, int s)
:target{t}, start{s} {}
};
struct StringGetter {
string *target = nullptr;
const int start = 0, len = 0;
explicit StringGetter (string *t, int s, int l)
:target{t}, start{s}, len{l} {}
};
// NB writes to an int target, not uint, fitting AisXXX classes member vars
struct UIntGetter {
int *target = nullptr;
const int start = 0, len = 0;
explicit UIntGetter (int *t, int s, int l)
:target{t}, start{s}, len{l} {}
};

protected:
int num_bits;
int num_chars;
Expand All @@ -429,6 +473,19 @@ class AisBitset : protected bitset<MAX_BITS> {
// This field is also used to determine the number of remaining bits after the
// last read position.
mutable int current_position;

// Memory safe fetchers for values stored in AisBitset.
// These depend on the above 'getter' structs to define the sub-sequence
// location in the AisBitset to fetch from, the value type to read, and the
// address to write the fetched read value to.
//
// If the requested sub-sequence falls inside the current AisBitset (success),
// sets *getter.target with the fetched value and returns 0.
// Otherwise (i.e. requested out of bounds), if asserts are enabled dies on an
// assert, or if not returns -1 and doesn't modify *getter.target.
int GetValue(BitGetter getter) const;
int GetValue(UIntGetter getter) const;
int GetValue(StringGetter getter) const;
};

class AisMsg {
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55 changes: 34 additions & 21 deletions src/libais/ais20.cpp
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Expand Up @@ -18,58 +18,71 @@ Ais20::Ais20(const char *nmea_payload, const size_t pad)
status = AIS_ERR_BAD_BIT_COUNT; return;
}

using UIGet = AisBitset::UIntGetter;

// 160, but must be 6 bit aligned
assert(message_id == 20);

bits.SeekTo(38);
spare = bits.ToUnsignedInt(38, 2);

offset_1 = bits.ToUnsignedInt(40, 12);
num_slots_1 = bits.ToUnsignedInt(52, 4);
timeout_1 = bits.ToUnsignedInt(56, 3);
incr_1 = bits.ToUnsignedInt(59, 11);
status = bits.GetValues(UIGet{&spare, 38, 2},
UIGet{&offset_1, 40, 12},
UIGet{&num_slots_1, 52, 4},
UIGet{&timeout_1, 56, 3},
UIGet{&incr_1, 59, 11});
if (!CheckStatus()) return;

if (num_bits == 72) {
spare2 = bits.ToUnsignedInt(70, 2);
status = bits.GetValues(AisBitset::UIntGetter{&spare2, 70, 2});
if (!CheckStatus()) return;

assert(bits.GetRemaining() == 0);
status = AIS_OK;
return;
}

group_valid_2 = true;
offset_2 = bits.ToUnsignedInt(70, 12);
num_slots_2 = bits.ToUnsignedInt(82, 4);
timeout_2 = bits.ToUnsignedInt(86, 3);
incr_2 = bits.ToUnsignedInt(89, 11);
status = bits.GetValues(UIGet{&offset_2, 70, 12},
UIGet{&num_slots_2, 82, 4},
UIGet{&timeout_2, 86, 3},
UIGet{&incr_2, 89, 11});
if (!CheckStatus()) return;

// 100 bits for the message
// 104 is the next byte boundary
// 108 is the next 6 bit boundary -> 18 characters
if (num_bits >= 100 && num_bits <=108) {
spare2 = bits.ToUnsignedInt(100, bits.GetRemaining());
status = bits.GetValues(UIGet{&spare2, 100, bits.GetRemaining()});
if (!CheckStatus()) return;

status = AIS_OK;
return;
}

group_valid_3 = true;
offset_3 = bits.ToUnsignedInt(100, 12);
num_slots_3 = bits.ToUnsignedInt(112, 4);
timeout_3 = bits.ToUnsignedInt(116, 3);
incr_3 = bits.ToUnsignedInt(119, 11);
status = bits.GetValues(UIGet{&offset_3, 100, 12},
UIGet{&num_slots_3, 112, 4},
UIGet{&timeout_3, 116, 3},
UIGet{&incr_3, 119, 11});
if (!CheckStatus()) return;

// 130 bits for the message
// 136 is the next byte boundary
// 138 is the next 6 bit boundary -> 23 characters
if (num_bits >= 130 && num_bits <= 138) {
// Makes the result 8 bit / 1 byte aligned.
spare2 = bits.ToUnsignedInt(130, bits.GetRemaining());
status = bits.GetValues(UIGet{&spare2, 130, bits.GetRemaining()});
if (!CheckStatus()) return;

status = AIS_OK;
return;
}

group_valid_4 = true;
offset_4 = bits.ToUnsignedInt(130, 12);
num_slots_4 = bits.ToUnsignedInt(142, 4);
timeout_4 = bits.ToUnsignedInt(146, 3);
incr_4 = bits.ToUnsignedInt(149, 11);
status = bits.GetValues(UIGet{&offset_4, 130, 12},
UIGet{&num_slots_4, 142, 4},
UIGet{&timeout_4, 146, 3},
UIGet{&incr_4, 149, 11});
if (!CheckStatus()) return;

spare2 = 0;

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67 changes: 43 additions & 24 deletions src/libais/ais26.cpp
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Expand Up @@ -24,83 +24,102 @@ Ais26::Ais26(const char *nmea_payload, const size_t pad)
}

// TODO(schwehr): Check for off by one.
const size_t comm_flag_offset = num_bits - 20;
const int comm_flag_offset = num_bits - 20;

if (num_bits < 52 || num_bits > 1064) {
status = AIS_ERR_BAD_BIT_COUNT;
return;
}

assert(message_id == 26);

bits.SeekTo(38);
const bool addressed = bits[38];
use_app_id = bits[39];

using BitGet = AisBitset::BitGetter;
using UIGet = AisBitset::UIntGetter;

bool addressed = false;
status = bits.GetValues(BitGet{&addressed, 38},
BitGet{&use_app_id, 39});
if (!CheckStatus()) return;

if (addressed) {
dest_mmsi_valid = true;
dest_mmsi = bits.ToUnsignedInt(40, 30);
status = bits.GetValues(UIGet{&dest_mmsi, 40, 30});
if (!CheckStatus()) return;

if (use_app_id) {
if (num_bits < 86) {
status = AIS_ERR_BAD_BIT_COUNT;
return;
}
dac = bits.ToUnsignedInt(70, 10);
fi = bits.ToUnsignedInt(80, 6);
status = bits.GetValues(UIGet{&dac, 70, 10},
UIGet{&fi, 80, 6});
if (!CheckStatus()) return;
}
// TODO(schwehr): Handle the payload.
} else {
// broadcast
if (use_app_id) {
dac = bits.ToUnsignedInt(40, 10);
fi = bits.ToUnsignedInt(50, 6);
status = bits.GetValues(UIGet{&dac, 40, 10},
UIGet{&fi, 50, 6});
if (!CheckStatus()) return;
}
// TODO(schwehr): Handle the payload.
}

bits.SeekTo(comm_flag_offset);
commstate_flag = bits[comm_flag_offset];
sync_state = bits.ToUnsignedInt(comm_flag_offset + 1, 2); // SOTDMA and TDMA.
status = bits.GetValues(UIGet{&commstate_flag, comm_flag_offset, 1},
// SOTDMA and TDMA
UIGet{&sync_state, comm_flag_offset + 1, 2});
if (!CheckStatus()) return;

if (!commstate_flag) {
// SOTDMA
slot_timeout = bits.ToUnsignedInt(comm_flag_offset + 3, 3);
slot_timeout_valid = true;
status = bits.GetValues(UIGet{&slot_timeout, comm_flag_offset + 3, 3});
if (!CheckStatus()) return;

switch (slot_timeout) {
case 0:
slot_offset = bits.ToUnsignedInt(comm_flag_offset + 6, 14);
slot_offset_valid = true;
status = bits.GetValues(UIGet{&slot_offset, comm_flag_offset + 6, 14});
if (!CheckStatus()) return;
break;
case 1:
utc_hour = bits.ToUnsignedInt(comm_flag_offset + 6, 5);
utc_min = bits.ToUnsignedInt(comm_flag_offset + 11, 7);
utc_spare = bits.ToUnsignedInt(comm_flag_offset + 18, 2);
utc_valid = true;
status = bits.GetValues(UIGet{&utc_hour, comm_flag_offset + 6, 5},
UIGet{&utc_min, comm_flag_offset + 11, 7},
UIGet{&utc_spare, comm_flag_offset + 18, 2});
if (!CheckStatus()) return;
break;
case 2: // FALLTHROUGH
case 4: // FALLTHROUGH
case 6:
slot_number = bits.ToUnsignedInt(comm_flag_offset + 6, 14);
slot_number_valid = true;
status = bits.GetValues(UIGet{&slot_number, comm_flag_offset + 6, 14});
if (!CheckStatus()) return;
break;
case 3: // FALLTHROUGH
case 5: // FALLTHROUGH
case 7:
received_stations = bits.ToUnsignedInt(comm_flag_offset + 6, 14);
received_stations_valid = true;
status = bits.GetValues(UIGet{&received_stations, comm_flag_offset + 6, 14});
if (!CheckStatus()) return;
break;
default:
assert(false);
}
} else {
// ITDMA
slot_increment = bits.ToUnsignedInt(comm_flag_offset + 3, 13);
slot_increment_valid = true;

slots_to_allocate = bits.ToUnsignedInt(comm_flag_offset + 16, 3);
slot_increment_valid = true;
slots_to_allocate_valid = true;

keep_flag = bits[comm_flag_offset + 19];
keep_flag_valid = true;

status = bits.GetValues(UIGet {&slot_increment, comm_flag_offset + 3, 13},
UIGet {&slots_to_allocate, comm_flag_offset + 16, 3},
BitGet{&keep_flag, comm_flag_offset + 19});
if (!CheckStatus()) return;
}

// TODO(schwehr): Add assert(bits.GetRemaining() == 0);
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